1. Field of the Invention
This invention relates to the transmission of digital information over cable pairs in the telephone plant, and in particular to the repeatered line equipment required for the transmission of such information where modified duobinary encoding is employed.
2. Description of the Prior Art
Initially, correlative systems found use in data transmission systems. Most commonly, bit rates in the order of 2400 and 4800 bps were used, and transmission was either over metallic wire pairs or carrier derived voice channels. In such applications, regeneration of the correlative level-coded signal was not a basic requirement.
PCM systems, such as the Western Electric T1 type, have been employed in commercial telephone service since the early 1960's. Each such system provides 24 voice-grade telephone channels. The system bit rate is 1.544 Megabits per second (Mb/s). Primary use of such systems has been over cable pairs in the exchange plant. In transmitting a 1.544 Mb/s digital signal over cable pairs, the signal becomes so degraded that restoration is required at intervals of about 6,000 feet. Regenerative repeaters have been used, and such repeaters are exemplified for prior-art systems by U.S. Pat. No. 2,996,578.
In order to double the transmission capability of the cable pairs, prior-art systems, which have been recently developed, have multiplexed two 24-channel PCM systems for transmission over an exchange cable pair in one direction of transmission. The bit rate is then increased to 3.152 Mb/s, more than double the rate for two systems. This is required because of pulse stuffing, which is employed in the time-multiplexing process. in essence, the capacity of the system is doubled because 48, rather than 24, channels are available for transmission over the repeater span. Because of this, the bit rate over the line is also doubled. For this reason, such conventional 48-channel systems require a new repeater which will operate at the new bit rate and over the greater frequency band equal to twice that required for the T1-type lines. Further, these conventional systems suffer from the problems of substantial additional crosstalk coupling which increases with frequency at the rate of 4.5 dB/octave because of the requirement for higher frequency transmission. Because of this and other factors, the number of 48-channel systems that may be accommodated in a single cable depends upon the number of pairs in the cable and whether screened cable is employed. In certain applications, the use of two different cables, one for each direction of transmission, is the most desirable approach. Thus, such 48-channel conventional systems have limited applications, and in many instances cannot be used to retrofit existing 24-channel lines. In addition to the factors of crosstalk coupling loss, the cable loss is important with respect to the repeater spacing, i.e., the physical distance between successive tandem repeaters. The problems involved relate directly back to the higher bit rate employed for the transmission of the 48-channel PCM group. Engineering considerations for a 48-channel system, which employs a transmission frequency of about 3.152 MHz, are contained in the GTE LENKURT publication, Section 836-910-073, issue 1, November 1974, a copy of which is attached. The subject GTE LENKURT publication is incorporated herein by reference.
These problems were overcome by the present invention which provides for transmission of 48 voice-grade channels having an effective bit rate of 3.152 Mb/s, but only requires about the same bandwidth for transmission as is employed for a 24-channel system using a bipolar, PCM signal. Thus, the crosstalk noise problems at higher frequencies are minimized. Further, the invention provides a transmission technique which is a suitable 48-channel retrofit for the 24-channel lines.